Signaling system



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SIGNALING SYSTEM Filed July 17, 1942 10 Sheets-Sheet 9 r0 BAN/(S or SELECTORS //v l/E/V TOR N D. NEWBV A TTORNE V June 19441 N. D. NEWBY 2,350Q917 SIGNALING SYS-TEM Filed July 17, 1942 10 Sheets-Sheet l0 INVENTOR Al. 0. NEWS) A TTORNEV Patented June 6, 1944 UNITED STATES PATENT OFFICE Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application July 17, 1942, Serial No. 451,304

9 Claims.

This invention relates to signaling systems and particularly to telephone systems in which voice frequency signaling currents are employed.

Objects of the invention are the provision of more reliable signal transmitting and receiving means in systems employing voice frequency signaling and the prevention of false operation in response to voice currents or other interfering currents.

This invention is a signaling system comprising means for transmitting signals comprising current of a first frequency followed by current of a second frequency or for transmitting signals comprising current of the second frequency followed by current of the first frequency, and means for responding to such signals without responding to voice currents or other interfering currents. Similar transmitting and receiving means are connected to both ends of a toll telephone line, different frequencies being used for signaling in each direction thereover. Means are provided for opening the talking connection over the associated toll telephone line whenever signals are being transmitted or received thereover, the connection opening means being non-responsive to voice currents or other interfering currents having a signaling frequency component.

A feature of the invention is the provision of means for repeatedly transmitting the disconnect signal over a toll line until an acknowledgment signal is received from the other end of the line. Another feature is the provision of means for transmitting an acknowledgment signal over a toll line, responsive to a disconnect signal from the other end of the line, and for terminating the transmission of the acknowledgment signal when the disconnect signal ends.

In signaling systems in which one type of signal consists of an impulse of current of a first frequency characteristic followed by an impulse of current of a second frequency characteristic and another type of signal consists of an impulse of current of the second frequency characteristic followed by an impulse of current of the first frequency characteristic, it is necessary to introduce an interval of time between the transmission of a signal of one type and another signal of the same type in order to distinguish from a signal of one type immediately followed by a signal of the other type. Other features of the invention are the provision of signal transmitting means arranged to introduce a predetermined minimum interval of time between successive like signals and the provision of signal receiving means arranged to test for said predetermined interval between successive signals of like type, thereby to distinguish between a signal of one type followed by a signal of the other type and a signal of one type followed by a signal of the same type.

A clear and complete understanding of the invention will be facilitated by considering a system embodying the invention and its features, one such system being represented schematically in the drawings which form a part of this specification. The invention is not limited in its application to the particular system and circuit arrangements shown in the drawings but is applicable generally to any voice frequency signaling system.

Referring to the drawings:

Figs. 1 to 5, inclusive, represent a first toll ofiice comprising an operators position, an outgoing trunk circuit O'I'l, an incoming trunk circuit ITI a two-way trunk circuit TWTI, a toll line circuit TLC'I, a signal transmitting circuit ST! and a signal receiving circuit SRI;

' Figs. 6 to 10, inclusive, represent another toll office comprising toll line circuit TLCZ, signal transmitting circuit STZ, signal receiving circuit SR2, two-way trunk circuit TWT2, outgoing trunk circuit 0T2, toll route selector T82, and an incoming trunk circuit 1T2;

Fig. 11 shows the relative position of Figs. 1 to 10, inclusive, to form an operative arrangement.

Reference may be had to the patent to R. E. King et al., No. 2,209,777, granted July 30, 1940, for a detailed disclosure of the cord and operators position circuits provided in each of the two toll offices for interconnecting calling and called lines and trunks and controlling the completion of toll calls. The outgoing trunk circuits OTI and GT2, incoming trunk circuits ITI and 1T2, the two-way trunk circuits TWI'I and TWT2, and the toll route selector circuit TS2 are all similar to corresponding circuits in the aforementioned King et al. patent.

The toll line circuits TLCl and TLC2, and the associated signal transmitting and receiving circuits, ST! and ST2 and SRI and SR2, are arranged for voice frequency signaling over the associated toll lines; whereas in the aforementioned King et al. patent, composite signaling circuits are associated with the toll lines. The toll line circuit TLCI comprises a repeating coil 408, attenuation pads PM and P42, pad control relays M0 and 4| l, a directionally selective coil 42!), a cut-off relay 425, and a repeater termination consisting of condenser 423 and resistor 424 in series. The directionally selective coil 42s permits signaling current incoming over toll line T12 to pass to the signal receiver SRI and permits signaling current from transmitter STI to pass out over line T12, but coil 420 prevents the passing of any substantial portion of the signaling current outgoing from transmitter STI into the signal receiver SRI. The normally operated cut-off relay 425 is released to open the talking connection between toll line TH and the trunk circuits OT! and. ITI whenever signals are being sent out by transmitter STI or received by the signal receiver SR1.

The toll line circuit TLCZ is similar to the circuit TLCI, the reference characters of like elements in these circuits having the same tens and units di its.

The signal transmitter STI comprises relays 43l, 433 and 435 connected in series for control by seizure, selective and supervisory direct current signals transmitted by signal relay I49 of the outgoing trunk circuit OTI. Relays 432, 434, 438;: 44!, 444- and 445 ar controlled byrelays 43L 433 and 435 to transmit signals consisting of current of one of two signaling frequencies either followed or preceded by current of the other frequency for required intervals of time.-

Each of relays 435, 43B and 444 is a polarized relay having anormally energized biasing winding for normally holding the contacts in the position shown inthe drawings. Each of relays 44! and 445 has a normally energized operating winding which normally maintains the contacts in the position shown in the drawings; and has a normally energized biasing winding which isefiective to move the contacts to their alternate position when the circuit through the operating winding is opened. The tone supply sources 4' and'48'l maybeoscillators of known design, one generating oscillations at one of the two signaling frequencies and the other generating oscillations at the other of the two signaling frequencies employed for transmitting signals over toll line T12 to .thesecond toll office. Relay 45B and interrupter 459 are effective on outgoing calls to cause the. repeated transmission of the disconnect signal and on incoming calls to cause the repeated transmission of adisconnect acknowledgment signal. The signal. transmitter circuit ST2 is similar to the transmitter circuit STI, the reference characters of like elements in these circuits having the same tens and units digits, The two frequencies used for transmitting signals over'toll line T12 in one direction are different from the two frequencies used for transmitting signals over toll line TI2 in the other direction; for instance, the frequencies 680 cycles per second and 1615.cycles per second may be used in.

one direction and the frequencies 850 cycles per second andl870 cycles per second may be used in the other direction.

The signal receiver circuit SR! comprises an input transformer 5H1, an amplifier comprising vacuum tube 5E5, transformer 520, a'peak limiting amplifier comprising vacuum tube 525, tuned networks 530 and 545, rectifiers 533 and 5 .3, relay 534 for response to current of one of the two frequencies used .for. transmitting signals over toll line Tl2 from the second toll ofiice, and relay 544' for response to current of the other of these two frequencies. Although the directionally selective coil 420] prevents the passing of a large portionof signaling currents from transmitter STI to receiver SR1, further protection against response to outgoing signaling currents is provided by a suppression network tuned to the outgoing signaling frequencies connected across the left windings of transformer 5). This network comprises condenser 505 and inductor 506 in series with a filter consisting of inductor 551 and condenser 508 in parallel. In order to secure a si nal response, it is necessary that the signaling current of each frequency comprising a voice frequency signal incoming to the signal receiver be somewhat stronger than the sum of all other currents present at the time, and that current of the second of the two frequencies constituting a signal follow the current of the first frequency within a predetermined interval of time. Only if both of these conditions exist, will a signal operation be effected; and for this reason voice currents or other interfering currents will not effect a signal operation. A high resistance element 52!, shunted by a small condenser 522, is connected in series with the grid of tube 525 and a non-linear impedance element 523, for instance, a piece of silicon carbide, is connected across the right windings oftransformer 52!] to limit the voltage impressed on the grid circuit of tube 525. If the desired limiting action is obtainable by the use of the resistor 52I and condenser 522 alone, the element 523 may be omitted; or if the desired limiting action is obtainable by the use of the non-linear impedance element alone, the resistor MI and condenser 522 may be omitted. Although the plate current of tube 525 never exceeds-a definite limiting value, the relative proportion of the various energy components in the output is at all times the same as that of thesecurrent equal in value to somewhat more than half the maximum output of tube 525. In other words, the energy of signaling frequency Fl must exceed the sum of all other energy components in the plate circuit to efiect the operation of relay 534; and the energy of signaling frequency F2 must exceed the sum of all other energy components to effect the operation of relay 535; and, therefore, both of these relays cannot be operated at the same time. Relays 535, 531, 545, 541, 550, 560, 56l, 552, 510, 515, 580, 585, 590, 591, 592, 553 and 594 convert the incoming voice frequency signals into direct current seizure, selective and supervisory signals which effect desired operations in establishing, maintaining, and releasing desired connections. Relays 535 and 545 are slow-to-release relays which are directl controlled by relays 534 and 544. If relays 534 and 535 are operated responsive to current of signal ing frequency Fl, relay 535 remains operated for a predetermined interval of time after relay 534 result from the successive operations of relays 544 and 534. These protections against false signal operation will be further explained in describing the operations involved on calls over toll line TI2. Each of relays 534, 544, 537, 541, 550, 560, 513 and'585 is a polarized relay which has a normally energized biasing winding for normally maintaining the contacts in the position shown in the drawings. Relay 515 is a polarized relay having a normally energized operating winding which normally maintains the contacts in the position shown in the drawings and which has a normally energized biasing winding which effects the operation of the contacts to their alternate position when the circuit through the operating winding is opened. Relays 58B and 585 are provided for holding the cut-off relay 425 released for about .550 second after the end of each incoming signal, this arrangement being necessary,

in case the toll line TI2 is connected by a cord CD to another toll line arranged for voice frequency signaling, to prevent false signal operations at the distant end of the other toll line. The delay in the reoperation of relay 425 also prevents interference with disconnect signals. Relays 59I, 592, 593 and 594 are provided for checkingincoming disconnect or disconnect acknowledgment signals as'hereinafter described. On outgoing calls relay 590 'is not operated and the operation and locking of relay 58B is controlled by relays 560 and 510; but on incoming calls relay 590 is operated to place the operation and locking of relay 589 under the control of relays 43L 432 and 434 of the signal transmitt'ng circuit STI. With relay 599 normal, the operation of relay 450 of signal transmitting circuit STI is under the control of relays 43I, 432 and 434 and the locking of relay 450 is under the control of relays 59I and 592 of the signal receiver SRI; but with relay 590 operated. the operation of relay 450 is under the control of relays 59I, 592 and 533 and relay 459 has no locking circuit. The signal receiving circuit SR2 is similar to the circuit SRI, the reference characters of like elements in these circuits hav ng the same tens and units digits. V

The abovementioned network comprising condensers 555 and 508 and-inductors 505 and 5t? suppresses the outgoing signaling'frequencies but transients produced at the beginning and the end of a signal impulse may be passed to the receiver and tend to cause false operation and interference with response to incoming signals. By using a band-pass filter tuned to the incoming signaling frequencies in place of the band suppression filter, the outgoing signaling currents could not cause false operation of; the receiver. While the transients produced in an arrangement using a band-pass filter are not ashigh as those pro duced by the suppression network, they are of longer duration. Thus,- in' either case the transients may be high enough and long enough to cause interference unles's the transients are reduced. Such a reduction may be effected by signal wave shaping or by adding a bandpass fil er tuned to the outgoing'signaling frequencies inserted between the secondary winding of transformer 429 and theback contacts of the cut-off rela 42501 the signal transmitter STI. If the filter between the directionally selective coil 42?! and the signal receiver SRI is a band el mination filter, the signaling frequencies passed by the sending band-pass filter will be entirely rejected if the band width of theband-pass filter is equal to or less than the band widthof the elimination filter. If the filter between the coil 420 and the signal receiver SRI is a band-pass filter, as above suggested, all frequencies within the pass band of the receiving filter are attenuated by the sending band-pass filter thereby preventing false operation and interference with the receiver.

Further explanation of the invention will be facilitated by describing in detail the circuit operations involved on a call over toll line TIZ. Assume first that the plug P2 of a cord CD is inserted in jack J2 of the trunk circuit OTI in Fig. 1, thereby closing a circuit from battery in the operators position circuit, throughthe sleeve conductor of the cord CD, as fully described in the aforementioned patent to King et al., through the sleeve conductor of jack J 2, a back contact of relay I06 of trunk circuit .OTI, and through the upper winding of relay IM'to ground. Relay I94 operates and closes a circuit for operating relay I33. Relay I33 locks under control of relay I 36 and closes a circuit for operating relay Hi3. Relay Hi! closes a circuit for lighting the busy lamp L2 associated with jack J2, closes circuits for operating relays I24, I36 and I51; closes a circuit including the inner lower contact of relay I52 for operatively energizing the upper winding of relay E40; closes a circuit through the middle biasing winding of relay I40 in series with ballast lamp I4I; connects ground to conductor I68 and disconnects the winding of relay I526 from conductor Hi8; disconnects conductor I18 from con duct-or I 59 and the windings -of relays 303 and 3&5 of the two-way trunk circuit TWTI'; and connects conductor I'll] to the winding of relay I65. The operation of relay I24 closes circuits for operating relays H2 and I20, connects ground of relays 395 and 303 from conductor I69. The

operation of relay I 36 opens the locking circuit of relay I33, but'relay" I33 holds under control of relay'IM. Rela I 33 is'slow to release and relay 136 is slow to operate so as to prevent the momentary release of relay I49 if relay I40 operates before the plug P2 is fully seated in jack J2. The operation of relay I40 closes a circuit from battery through resistor I42, front contact of relay I40, conductor I64, a back contact of'relay 456, through the windings of relays 43I, 433" and 435 of signal transmitting circuitSTI to initiate the transmissioncf a seizure signal over toll line T52. The operation of relay I I2 opens the normally closed circuit for operating pad control relay 4, closes a circuit for holding the busy lamp L2 lighted, and further opens the connection between conductors I69 and I'm. The operation of relay I20 disconnects relay I34 and conductor I6? from the windings of retard coil 5 63 and conductors l5! and IE2 and connects these conductors tlu'oughthe windings of relays I23 and i56 and conductor use to the winding of pad control relay 4I I; The operation of rela I36 opens a normally closed short circuit around the winding of relay I52 and closes a short circuit around resistor I53. Relay I52 is jthereupon operated in'a circuit through resistor J54, inner upper back contact of relay I32}, conductor lili, through the tip conductor of jack J2 and plug P2, thence to battery in the operators position circuit as described in the aforementioned King et al. patent. The operation of relay I52 closes a circuit for operating relay I5I and opens the operating circuit of relay I5I. Relay i5I locks under control of relay I36 and connects the operating, upper winding of relay I49 through a back contact of relay I30, conductor I92, the ring conductors of jack J2, plug P2 and cord CD to battery in the operators position circuit. When the dial key (not shown) is operated, the impulse contacts of the dial are connected in series with the operating winding of relay I40 as described in detail in the aforementioned King et al. patent; and, when the called toll route number is dialed, the impulse relay I40 operates and releases in response to each dial impulse to repeat the impulses to the signal transmitter STI as hereinafter described.

The aforementioned closing of the circuit through the front contact of relay I43 and wind ings of relays 43I, 433 and 435, in response to insertion of plug P2 in jack J2, causes the operation of these relays. Relay -43I closes a circuit for operating the slow-to-release relay 432, and causes the release of the normally operated cut-off relay 425. The circuit in which relay 425 is normally operated is traced from ground at a back contact of relay 450, the back contact of relay 43 I, through back contacts of relays 432 and 434, winding of relay 425, another back contact of relay 450, conductor 500, and through back contacts of relays 585 and 550, to battery. The release of relay 425 opens the connection between the talking conductors I6I and IE2 of outgoing trunk circuit OTI and repeating coil 409, and connects the left windings of repeating coil 429 to the repeating coil 400. The operation of relay 433 closes a circuit from ground through the left contact of relay 445, right windings of repeating coil 429, back contact of relay 444, front contact of relay 433, and through the tone source 4II to ground. I The operation of relay 432 connects tone source 4' to the back contact of relay 444, the front contacts of relays 432 and 433 being connected in parallel. The alternating potential of frequency FI generated by the tone source 4'" initiates signaling current of frequency FI through repeating coils 429 and 400, to transmit the first part of a seizure signal over toll line TI2 to the signal receiver SR2 in the second toll office. The aforementioned operation of relay 435 closes a circuit for energizing the lower wind ing of relay 438 and opens the normally closed circuit through the lower winding of relay 44I. Upon energization of its lower winding, relay 438 opens its left contact and closes its right contact. When the circuit through the lower winding of relay 44I is openedby the operation of either or both of relays 435-and 438, condenser'439 is charged in series with resistor 440, the charging current being effective to hold relay 44l operated and thereby delay the operation of relay 444 for about .030 second after the operation of relay 433 initiates the signaling current of frequency FI. When the charging current is no longersufficient to maintain the operative energization of the lower winding of relay 44!, the normally energized upper winding of relay 44I effects the opening of the left contact and the closing of the right contact of this relay, there-V.

by opening the energizing circuit through the upper winding of relay445 and closing an operating' circuit through the upper winding 'of. relay 444. The operation of relay 444 disconnects the right windings of repeating coil 429 from tone source 4II, and'connects the right windings of repeating coil 429 to tone source 48I. Since the closing of the operating circuit of relay 444 is delayed by the charging of condenser 439, the current of frequency Fl through coils 429 and 400 and toll line TI 2 continues for a minimum of .030 second before the operation of relay 444 terminates the signaling current of frequency FI and initiates a signaling current of frequency F2. The upper winding of relay 445 continues to be energized, after the operation of relay 44 I, by current charging condenser 442 in series with resistor 443. The opening of the left contact and closing of the right contact of relay 445 is thereby delayed for an interval of approximately .050 second, during which interval there is a current of frequency F2 through coils 429 and 400 and toll line TI2. The opening of the left contact of relay 445 ends the signaling current of. frequency F2, and the closing of the right contact of relay 445 closes a circuit, including a back contact of relay 434, for energizing the winding of the cut-off relay 425. Reoperation of relay 425 disconnects repeating coil 429 from repeating coil 400 and reconnects coil 400 to conductors IBI and I62.

Thus a seizure signal, consisting of current of frequency of Fl for an interval of not less than .030 second followed immediately by current of frequency F2 for an interval of about .050 second. is transmitted over toll line TI2 to the second toll oflice, through repeating coil B00, pads P6I and P62, the left windings of directionally selectiv coil 620, conductors IOI and 102, condensers I03 and I04 and the right windings of transformer H0. The signal energy is applied through transformer H0 to the grid of amplifying tube H5, is amplified by tube II 5, and is applied through transformer I20 to the grid of tube I25. The resistor I2I in the grid circuit of tube I25 eifects a drop in potential which limits the amplitude of the output voltage. The network offers substantially infinite impedance to current of frequency FI, so that the signaling current of this frequency in the plate circuit of tube I25 is applied to and rectified by the full-wave rectifier I33; and the rectified current operatively energizes the upper winding of relay I34. When the signaling current of frequency Fl ceases, rela I34 releases. The network I40 offers substantially infinite impedance to current of frequency F2; and the signaling current of this frequency in the plate circuit of tube 125, immediately following the current of frequency F2, is applied to and rectified by the rectifier I43; and the rectified current operatively energizes the upper winding of relay I44. When the current of frequency F2 ceases, relay I44 releases. Since the current in the plate circuit of tube I25islimited to a value which is sufficient for operating relay I34 only when substantially all of the current in line TI 2 is of frequency FI and which is su-filcient for operating relay I44 only when substantially all of the current in line LI is of frequency the operation of either of these relays, when. voice or other interfering currents are present, is prevented. The operation of relay I34 in response to the impulse of current of frequency FI closes a circuit for operating relay I35 and closes a circu t through rectifier I35, the middle w nding of relay I59. upper winding of relay I31 and re sistor I39, to battery. Since ground is normally connected through back contacts of re lays I31 and I45, to resistor I38, the operation of relay I34 is ineffective to energize the middle winding of relay I50 and upperwinding of relay 131. When the current of frequency F'I ceases, relay I34 releases but relay 135 is a slowto-release relay and remain operated for approximately .020 second after the release of relay 134. The operation of relay 144, in response to signaling current of frequency F2, immediately following the current of frequency Fl, closes a circuit for operating relay I45 and closes a circuit through rectifier 148, the lower winding of relay I58, the upper winding of relay 141 and resistor 148. Since the operation of relay 144, in response to a seizure signal, occurs before relay 135 has released, the current in the circuit through rectifier 143 effects the operation of relays 141 and 150. The operation of relay 158 opens the normally closed energizing circuit of cut-01f relay 925, this circuit being traced through back contacts of relays .150 and I85, conductor 189, a back contact of relay 650, winding of relay 625, and through back contacts of relays 634, 832 and 93L to ground at another back contact of relay 658-. The operation of relay I41 prevents the connection of a short-circuiting ground to resistor 148 when relay 135 releases, and closes a circuit from ground through conductor 149, a back contact of relay 1E2, conductor 159 and the lower Winding of relay 169 to battery. Relay 1 50 operates, closing a circuit from ground through its left contact, winding of relay 19!, conductor 910, lowermost back contact of relay 9!!! of outgoing trunk circuit T2, lower back contact of relay 9I2, conductor 959, back contact of relay 804 of two-way trunk circuit TWT2, back contact of relay 881, and through the winding of relay 803 to battery. Relays I and 883 are operated by the current in this circuit. The operation of relay 18! closes a circuit for operating relay 192 and connects ground to conductor 1% to maintain the energization of the lower winding of relay 1-50. Relay 192 looks under control of relays 198 and 193, connects ground to conductor 159, and connects the right contact of relay 160 through resistor 153 and condenser 164 to conductor 139 in preparation for the receiving of succeeding signals. When the current of frequency F2 ends, relay 144 releases, causing the release of relays 145, 141 and 158, the release of relay 145 occurring about .020 second after the release of relay 144. The release of relay 150 closes the circuit for operating cut-off relay "625; and relay 625 closes the talking connection between toll line TI 2 and conductors 96! and 962.

The aforementioned operation of relay 803, in response to the seizure signal, connects ground to conductor 823 to mark the two-Way trunk circuit TWTZ and associated toll line TI2 busy in the banks of the toll route selectors. This ground potential is further connected through conductors 823 and 968, back contacts of relays 92-4 and 9! 0 of the outgoing trunk circuit 0T2 to the winding of relay 999. Relay 908 operates, connects battery to the sleeve of jack J9 as a busy indication, and closes a circuit for operating relay 9I2. Relay 9I2 closes a circuit for lighting busy lamp L9 and opens the normally closed circuit for operating pad control relay Bl I The aforementioned operation of relay 883 disconnects conductors Bill and 802 from conductors 82! and 822 leading to terminals in the banks of toll route selectors, and closes a circuit for operating relay 885 of trunk circuit TWTZ and line relay 851 of toll selector TSZ. This circuit is traced from ground through the front contact of relay 169, winding of relay 'I8I, conductor 910, back contact of relay 918, front contact of relay 906, conductor 969, back contact of relay 804, winding of relay 805, inner upper front contact of relay 803, conductor 961, thence in simplex through resistors 92] and 922, inner back contacts of relay 920, windings of retard coil 960, outer back contacts of relay 938, conductors SM and 802, outer front contacts of relay 803, conductors I and 852, back contacts of relay 890 of selector T32, resistors 86I and 892, to and through resistor 883 and another back contact of relay 858, through the winding of relay 851 to battery. The operation of relay 805 closes a circuit through resistor 888 for holding relay 803 and closes a circuit for operating relay 881. Relay 801 closes a holding circuit for relay 883 and connects ground to conductor 853 thereby operating relay I98 of signal receiver SR2. Relay disconnects the winding of relay 180 from the contact of relay 110, to prevent the operation of relay 189 in response to signals incoming over line TI2, closes a locking circuit for relay 162 to ground at a back contact of relay 193, and connects relay 180 to conductor 655 for operation as hereinafter described during the transmission of signals over line TI2 to the calling office. Relay 891 is a slow-to-release relay in order to maintain the connection of ground to conductor 853 during the response of relay 895 to dial impulses as hereinafter described. The operation of relay 851 closes a circuit for operating the slow-to-release relay 858. Relay- 858 connects ground to conductor 853, and connects ground to conductor 859 leading to the alarm circuit which operates in usual manner to operate an alarm device if the selector TS2 fails to complete its selective operations within a predetermined interval of time. 7

No further operations take place until the calling operator dials the desired toll route number and also the called subscribers number if the called line is in an automatic office. Relay I48 of outgoing trunk circuit OT! responds tothe dial impulses, opening and closing the circuit through conductor I64 and the windings of relays 43 I', 433 and 435 in response to each opening and closing of the impulse contacts of the dial. The release of relay 43I opens the operating circuit of relay 432, but relay 432 is slow in releasing and remains operated during the transmission of dial impulses. With relay 43I released and relay 432 operated, a circuit is closed in response to the first impulse of each train for operating relay 434; and since relay 434 is slow in releasing, it remains operated until relay 433 reoperates, after receiving the last impulse of the train. The operation of relay 434 causes the release of the cut-off relay 425 to disconnect coil 489 from conductors I8! and I62 during the transmission of dial impulses. The release of relay 435 opens the circuit through the lower winding of relay 438 and closes a circuit including the right contact of relay 438 for energizing the upper winding of relay 445. Relay 438 is held operated, after relay 435 releases, by current charging condenser 436. The energization of the upper winding of relay 445 effects the closing of its left contact; and, since relay 434 is operated and the right contact of relay 444 is closed, the right windings of coil 429 are connected to tone source 48I to initiate a current of frequency F2 in line TI2. As soon as the current charging condenser 436 is no longer suificient to maintain the closure of the right contact of relay 438, (about .030 second after relay 435 releases), the lower winding of relay MI is again energized through the left contacts of relays 438 and'435.

41! thereto, thereby ending the current of frequency F2 through coils 429 and 400 and toll line Tl2 and starting current of frequency Fl. When relay 435 reoperates, at the end of the first dial impulse, it opens the circuit for energizing the lower winding of relay 4M and closes the circuit for energizingthe lower winding of relay 433. Relay 438 immediately actuates its contacts but the actuation of the contacts of relay 44! is delayed while condenser 439 is charging; When relay 441 k actuates its contacts, about .030 second after relay 435 reoperates, the upper winding of relay 444 is energized; and the coil 429 is disconnected from tone source 41! and again connected to tone source 48l, thereby ending the current of frequency FI and initiating current of frequency F2. Each release and reoperation of relay 435 in response to succeeding dial impulses in the "same trainefiects the above-described cycle of operations whereby an impulse of current of fre- U quency F2 is transmitted, followed by an impulse of current of frequency Fl, through coils 429 and 403 and toll line TI2 for each'dial impulse. At the end of the last impulse of the train, the current of frequency F2 continues for about .05 second while condenser 442 is being charged, whereupon the contacts of relay 445 are actuated to open the circuit through the right windings of coil 429. Relay 434 releases when relay 431 remains operated at the end of a train of impulses, thereby causing the reoperation of cut-off relay 425.

Dialing may start within .080 second after the plug P2 is inserted in jack J2. In this case, relay 445 has not been'operated to 'end the current of frequency F2 of the seizure signal, the release of relay 43l and operation of relay 434 maintain the de'energization of relay 425, and the release of relay 435 prevents the d'eenergization of the upper winding of relay 445, for about .030 second while condenser 435 is charging and until relay 4'38 opens its right contact. Thus there is no interval between the 'current of frequency F2 or the seizure signal and the current of frequency F2 for the first dial impulse.

Each succeeding train of dial impulses effects the operation of the signal transmitter STI in like manner to transmit v'oi'ce frequency impuises over toll line Tl2 to the receiver SR2 in the second toll office. The signal receiver SR2 responds to each train of dial impulses, the fir'st'tra'in being effectiveto control the selective operation of selector T82. The current of frequency F2, responsive tothe first opening'o'f the impulse contacts of the dial, causes the operation of relay 144. Relay I44 closes a circuit for operating relay 145 and'closes a circuit through rectifier I46 and windings of relays 141 and 150. Since relay 135 is not then operated, the windings of relays 141 and 150 are short-circuited through bacl; contacts of relays 1 and 1-41, When the current of frequency F2 through toll line TI 2 ceases and current of frequency 'Fl 5 begins, relay 14 4 releases and relay 1'34 operates. Being "slow in releasing,

" w 145 remains "operated for about 1020 second after relay 144 releases; and the operation of relay 134, while relay 145 is operated, effects the operation of relays I35, 131 and 150. The operation of relay 15%] causes the release of cut-off relay 625 to disconnect coils 600 and 620 from talking conductors 9d! and 932; and the operation of relay 131 prevents the short-'circuiting of its upper winding, when relay 145 releases, and coin nects ground to conductor 139, thereby energizing the upper winding of relay 110. The energizatioh of the upper winding of relay 114 effects the opening of its left contact'and the closing of its right contact. The closing of the front contact of relay 110 is Without effect, since relay I is operated; but the opening of the left contact of relay opens the circuit through the upper winding of the normally operated relay 115, the deenergization 'of this winding being delayed for about .030 second while condenser 11! is being charged in' series with resistor 112. The deenergization of the upper winding of relay causes the opening of the left contact and closing of the right contact of this relay; whereby ground is disconnected from conductor I16 and connected to conductor 111. The disconnection of ground from conductor "Ii'6 effects an increase in the curre'nt'through the upper Winding "of relay to cause relay 160 to open its left contact to release relays 15!, 305 and 851 and close its right contact to discharge condenser I64. The connection of ground to conductor 111 operatively energizes the winding of relay 19i; but relay 192 is not energized. since its winding is short-circuited through its own inner lower back contact. Relay 19! looks under control of relay 162. When the current of frequency Fi ends, relays 734, and I31 release, the release of relay 135 being delayed for .020 second. The release of relay 1'31 disconnects ground from conductor 139 but the upper winding of relay "I10 remains energized for about .030 second .while condenser 164 is being charged in series with resistor 163. When the charging current is no longer effective -to operatively energize the upper winding of relay 110, the biasing winding effects the opening of the right contact and closing of the left contact of this relay. The upper winding of relay 115 is thereby energized-to-effect the disconnection ofground from conductor 111 by the opening of the right contact of relay 115 and the reconnection of ground to conductor 116 by the closing of the left contact of relay 115. The disconnection of ground from conductor =111 opens the short circuit around the winding of relay I92, whereupon this winding is energized in series with resistor 196, through the 100kiIlg"COI1- tact of relay 19l to ground at a front contact of relay 162. The reconnection of ground to conductor 116 decreases tlie'current through the upper windingof relay 168 to cause the 'closing of the left contactofr'ela'y 160 and thereby reoperate relays 161, '805 and 851. The'current'of frequency F2, immediately following the current of frequency FI in line TIZ, causes the successive operations of relays '1-44 and 145, Since relay 13'5 ha's'not yet released, the operati'on'of relay 144 causes the operation of relay 1'41 and holds relay and the operation of relay 141 connects ground to conductor 1'49 thereby causing the operative energization'ofrel'ay 194. The operation of relay 19'4fin response to the current of frequency F2 following each dial impulse signaLcauses therelease ofrelays 191 and '192, thereby preventing two succeeding dial impulse sighals frorn being recognized asa dis'connect sig- -maintains the connection of ground to conductor 159. Each succeeding dial impulse in the train effects the reoperation of the relays of the receiver SR2 to effect a release and reoperation of relays 16I, 865 and 851. When the current of frequency F2, following the last impulse of the train ceases, the release of relay 144 causes the release of relays 145 and 156. The release of relay 156 closes the circuit for operating cut-off relay 625 to reconnect coils 666 and 625 to talking conductors 96I and 962. Relay 166 now maintains the connection of ground through the winding of relay 16I to conductor 916 to hold relay 865 of the two-way trunk circuit TWT2 and to hold the line relay 851 of selector TSZ until an idle trunk is seized in the level corresponding to the first digit dialed by the calling operator.

Since relay 861 of two-way trunk circuit TWT2 and relay 858 of selector TSZ are slow in releasing, these relays remain operated during the response of relays 865 and 851 to dial impulses. The release of relay 851 in response to the first impulse incoming over toll line TIZ closes a circuit for operating the vertical stepping magnet 861 and the slow-to-release relay 869 in series. The operation of magnet 861 steps the brushes 81L 812, 813 and 814 up the first level of the terminal bank. The vertical ofi-normal springs 864 are actuated when the shaft is moved out of normal position, closing a circuit including the front contact of relay 869 and conductor 853 for operating relay 865. Relay 865 looks, through the back contact of rotary stepping magnet 866 and a front contact of relay 865, to ground at a back contact of relay 866. When relay 851 reoperates at the end of the first impulse, magnet 861 releases but relay 869 is slow in releasing and remains operated until all of the impulses in the train created by the dialing of the first digit have 865, back contact of relay 869, conductor 853, to

ground at the upper front contact of relay 858. The operation of magnet 866 rotates the brushes one step, into engagement with the first set of terminals in the selected level. When magnet 866 opens its back contact, relay 865 releases. The release of relay 865 causes the release of magnet 866. If the terminal, with which test brush 813 is in engagement, is guarded by a ground potential, relay 865 reoperates when magnet 866 releases; the operating circuit being traced through the lower front contact of springs 864, back contact of magnet 866, lowermost back contact of relay 866 and test brush 813. The

reoperation of relay 865 causes the operation of the rotary stepping magnet 866, thereby advancing the brushes to the next set of terminals. The operation of magnet 866 releases relay 865; and

'the release of relay 865 releases magnet 866. Relay 865 and magnet 866 repeat this cycle of operations, to advance the brushes step by step,

until brush 813 encounters a terminal not marked by ground potential; whereupon, relay 866 is operated to seize the idle set of terminals. The circuit for operating relay 866 is traced from battery through the winding of relay 865, front contact of the vertical ofi-normal springs 864, back contact of magnet 866, winding of relay 866, lower back contact of the 11th rotary step springs 856, through conductor 853 to ground at the upper front contact of relay 858. Relay 865 is marginal and does not reoperate in series with the winding of relay 866. Relay 866 disconnects the winding of relay 851 from conductors l and 852 and connects these conductors to brushes 8H and 812 thence to the trunk or selector circuit connected to the selected set of terminals. The release of relay 851 causes the release of relay 858 but ground is not thereby disconnected from conductor 853 since this conductor is connected to ground at the lower front contact of relay 861.

Assume first that the call is routed to an operator in the second toll oflice, that the terminals, in the level selected in response to the first digit dialed by the calling operator, are connected to incoming trunk circuits and that the terminals 816, connected to the trunk circuit 1T2, are selected by the trunk hunting operation of the selector TS2. In this case, the operation of relay 866 closes a circuit for operating the line relay I665 of trunk circuit 1T2. This circuit is traced from battery through the winding of relay I665, thence in simplex through; resistors I6I5,v I6I6 and Hill, the windings of retard coil I666, conductors I6! and I662, brushes 81I and 812, front contacts of relay 866, conductors 85I and 852, front contacts of relay 863, conductors 8M and 862, back contacts of relay 936, windings of retard coil 966, back contacts of relay 926, through resistors 92I and 922, thence through conductor 961, inner upper front contact of relay 863, Winding of relay 865, back contact of relay 864, conductor 969, a front contact of relay 966, lowermost back contact of relay 9I6, conductor 916, winding of relay 16I, to ground at the left contact of relay 166 of signal receiver SR2. Relays 16I and 865 are thereby held operated in series with relay I665. With relay I6I9 non-operated, resistor I6I8 is short-circuited; and, since the resistance of resistors I6I5 and I6" together is equal to that of resistor I6I6, the simplex is balancecl so that pad control relay 934 of outgoing trunk circuit 0T2 remains non-operated. In the incoming trunk 1T2 the winding of pad control relay I666 is bridged across the end of the simplex to the calling operator to indicate that the toll operator in the called toll oflice is being signaled. The calling operator having completed dialing restores the dialing key to normal, thereby reconnecting the operators telephone through plug P2 and jack J2 to the outgoing trunk circuit OTI and effecting the operation of relay I36 in the manner described in the aforementioned King et a1. patent.

When the call is answered by insertion of the .plug of a cord in jack J I6, relay I644 is operated in a circuit through conductor I643 and the sleeve lease of the normally operated. cut-off conductor of jack J I0, to battery in the answer- 'ing operators position circuit. Relay I044 closes 'a circuit for operating relay I634. Relay I034 opens the repeater termination, comprising resistor I033 and condenser I032 normally bridged across the talking conductors I04I and I042, and closes a circuit for operating relay I03I. This circuit is traced through the lower front contactof relay I034, front contact of relay I005, conductor I003, brush 6T3, lowermost front contact .of relay 860, conductor 853, to ground at the lower front contact of relay 861. Relay I03I locks under control of relay I000 through conductor I003, independent of relay I634.

ground, through the outer upper front contact ofrelay I034, back contact of relay IllI2, front contact of relay I03I, conductor I004, brush 814, a front contact of relay 860, a back contact of the 11th rotary step springs 656 of selector TSZ,

and through conductor 854 to the winding of re- Relay 8 closes a circuit from battery lay 8! I. through resistor 942 of outgoing trunk circuit 0T2, conductor 966, front contact of relay 8i I, back contact of relay BIO, conductor 965, back contact of relay 940, conductor 964 through the windings of relays 63I, 633 and 635, in series, to ground. Relay 63I closes a circuit for operating the slow-to-release relay 632 and causes the rerelay 625. The release of relay 625 opens the connection between the talking conductors 96I and 962 and repeating coil 600 and connects the right windings of repeating coil 629 to the repeating coil 600. The operation of relay 633 closes a circuit from ground through the right contact of relay 645, left windings of repeating coil 62 9, back contact of relay 644, front contact of relay 633, and through the tone source 6' to ground. The operation of relay 632 connects tone source 6H to the back contact of relay 644, the lower front contact of relay 632 and front contacts of relays 633 and 634 being connected in parallel. The alternating potential of frequency F3 generated by the tone source 6'II initiates signaling current of frequency F3 through repeating calls 629 and 600 to transmit the first part .of an answering supervisory signal over toll line TI 2 to signal receiver SRI. The operation of relay 635 closes a circuit for energizing the lower winding of .relay 638 and opens a circuit including the right contact of relay 638 and the lower winding of relay 64!. Upon energization of its lower winding, relay 635 opens its right contact and closes its left contact. When the circuit through the lower winding of relay MI is opened by the operation of either or both of relays 635 and 638, condenser 639 is charged in series with resistor 640, the charging current being effective -to hold relay .64I operated and thereby delay the operation of relay 644 for about..030 second after the operation of relay 633 initiates the signaling current of frequency F3. When the charging current is no longer sufficient to maintain the operative energization of the lower winding of relay 64!, the normally energized upper winding of relay 64I effects the opening of the right contact of this relay thereby opening the energizing circuit through the upper winding of re- .lay 645 and closing an operating circuit through the upper winding of relay 644. The operation Qof relay 644disconnects theleft windings of re- 75 "5.34 releases.

peating coil 629 from tone source 6' and connects the left windings of repeating coil 629 to tone source68l. Since the closing of the operating circuit of relay 644 is delayed by the charging of condenser 639, the current of frequency F3 through coils 629 and 600 and toll line TI2 continues for a minimum of .030 second before the operation of relay 644 terminates the signaling current of frequency F3 and initiates a signaling current of frequency F4. The upper winding of relay 645 continues to be energized, after the operation of relay MI, by current charging condenser 642 in series with resistor 643. The opening of the right contact and closing of the left contact of relay 645 is thereby delayed for an interval of approximately .050 second, during which interval'there is a current of frequency F4 through coils 629 and 600 and toll line TI2. The opening of the right contact of relay 645 ends the signaling current of frequency F4 and the closing of the left contact of relay 645 closes a circuit including a back contact of relay 634 forenergizing the winding of the cut-off relay 625. The reoperation of relay 625 disconnects repeating coil 629 from repeating coil 600 and 'reconnects coil 600 to talking conductors 96I and densers 503 and 504 and the left windings of transformer 5I0.

The signal energy is applied through transformer 5H] to the grid of amplifier tube 5I5, is amplified by tube 5I5, and is applied through transformer 520 to the grid of tube 525.

Th resistor 52I in the grid circuit of tube 525 effects a drop in potential which limits the amplitude of the output voltage. The network 530 offers substantially infinite impedance to current of frequency F3 so that the signaling current of this frequency in the plate circuit of tube 525 is applied to and rectified by the full wave rectifier 533; and the rectified current operatively energizes the upper winding of relay 534. When the signaling current of frequency F3 ends, relay The network 540 offers substantially infinite impedance to current of frequency F4; and the current of this frequency in the plate circuit of tube 52 5, immediately following the current of frequency F3, is applied to and rectified by the rectifier .543; and the rectified current operatively energizes the upper winding of relay When the current of frequency F4 ceases, relay544 releases. Since the current in th plate circuit of tube 525 is limited to a value which is sufficient for operating relay 534 only when substantiallyall of the current incoming in line TI2 is of frequency F3 and which is sufficient for operatingrelay 544 only when substantially all'of the current in line TI2 is of frequency F4, the

operation of either of these relays, when voice or rectifier 566, the middle winding of relay 550, upper winding of relay 531 and resistor 538 to battery. Since ground is normally connected through back contacts of relays 537 and 545 to resistor 538, the operation of relay 534 is ineffective to energize the middle Winding of relay 550and upper winding of relay 531. When the currentof frequency F3 ceases, relay 534 releases but relay535 is a slow-to-release relay and remains operated for approximately .020 second after the release of relay 534. The operation of relay 544, in response to signaling current of frequency F4 immediately following the current of frequency F3, closes a circuit for operating relay 545 and closes a circuit through rectifier 546, the lower winding of relay 550, the upper winding of relay 547 and resistor 548. Since the operation of relay 544, in response to the answering signal, occurs before relay 535 has released, the current inthis circuit effects the operation of relays 541 and 550. The operation of relay 550 causes the release of the cut-off relay 425. The operation of relay 541 prevents the connection of a short-circuiting ground to resistor 546 when relay 535 releases, and closes a circuit from ground through conductor 549, a back contact of relay 562, conductor 559 and the lower winding of relay 560 to battery. Relay 560 operates closing a circuit from ground through its right contact, winding of relay 56I, conductor I'I0,'a front con-- tact of relay II 0, and through the winding of relay I05. Relays 56I and I05 are operated by the current in this circuit. The operation of relay 56I closes a circuit for operating relay 562 and connects ground to conductor 559 to maintain the energization of the lower winding of relay 560. Relay 562 connects ground to conductor 559 and connects the left contact of relay 560 through resistor 563 and condenser 564 to conductor 539 in preparation for the receiving of succeeding signals. Relay 562 does not lock operated since relay 590 is not operated. When the current of frequency F4 ends, relay 544 releases causing the release of relays 545, 541 and 550, the release of relay 545 occurring about .020 second after the release of relay 544. The release of relay 550 closes the circuit for operating cutoff relay 425 to close the connection between coil 400 and talking conductors I6I and IE2. The aforementioned operation of relay I opens the circuit through the lower winding of relay I04 thereby leaving the high resistance upper winding of relay I04 in series with the sleeve conductor of jack J2 and plug P2 so as to extinguish the operators supervisory lamp SL2 as an answering or off-hook signal.

While the answering operators telephone set is connected to trunk IT2, battery is connected through jack J I0 to both of conductors I MI and I042, thence through the windings of retard coil IOI0, through resistor I M I and relay IOI 2, conductor IOI3, through the upper back contact of relay, I035 and Winding of relay I02I to ground. Relay I02I operates closing a circuit for operating relay IOI9. Relay I M 9 disconnects pad control relay I009 and opens the short circuit around resistor IOI8 and short-circuits resistor IOII to unbalance the simplex circuit through conductors IO0I and I002 so as to efiect the operation of pad control relay 934 of outgoing trunk circuit 0T2. Relay 034 closes a circuit through conductor 963 for operating relay 6. Relay. 6 opens the circuit for operating relay 6I0; and with relay 6I0 released, pads FBI and P62 are included in the talking circuit. After the opere ators telephone set is disconnected from trunk IT2 and this trunk is connected through thecord to another trunk or toll line, relay I02I is-held operated if thisother trunk or toll line is a switching trunk or tributary line but is'released if it is a toll lineor intertolltrunk. The release of relay I02I causes the successiverelease of relays IOI9, 934 and 6H and the operation of relay 6I0to short-circuit the pads RBI and P62. If the intertoll trunk associated with the outgoing trunk circuit 0T2 is one which is not provided with transmission pads, relay 934 is not provided. Ifthe 'intertoll trunk associated with the outgoing trunk circuit 0T2 is one which does not require the short-circuiting of the pads of a line or trunk to which the incoming trunk circuit 1T2 is connected by the answering operators cord, resistors 92I and 922 of the outgoing trunk circuit 0T2 would be of such value as to unbalance the simplex circuit and thereby cause the operation of relay I009 of the incoming trunk circuit 1T2. I The operation of relay I 009 closes a circuit for operating relay I035. Relay I035 disconnects relay I02I from conductors I MI and I042 and connects battery through resistors I037 and I036 to conductors I04 I and I042 to hold operated the pad control relay of the trunk or line to which the trunk circuit H2 is connected by the answering operator's cord.

The talking connection between outgoing trunk circuit OTI and incoming trunk circuit 1T2 includes conductors IOI and, I02, front contacts of relay I30, conductors I6I and I62, front contacts of relay 425, resistors 42I and 422, repeating coil 400, toll line TI2, repeating coil 600, resistors 62I and 622, front contacts of relay 625, conductors 96I and 962, back contacts ofrelay 930, conductors 8TOI and 802, front contacts ofrelay 603, conductors 85I and 852, front contacts of relay 860, brushes 8' and 812, conductors IO0I and I002, condensers I050 and conductors I04I and I042.

If .the answering operator disconnects cord CD2 from jack J I0, relay I044 releases causing the release of relay I034 to disconnect ground from conductor I004 and effect therelease of relay 8| I. The release of relay. BI I" causes the release of relays 63I, 633 and 635 of signal transmitter ST2, thereby eifecting the transmission of an on-hook signal consisting of current of fre quency F4 followed by current of frequency F3. The release of relay 63I closes a circuit for operating relay 634 and closes a circuit for operating relay I80. The circuit for operating relay IQis' traced from the winding of relay 160, through a front contact of'relay I90, conductor 655, a front contact of relay 632, and through the back contact of relay 63I to ground at a back contact of relay 650. Relay 180 locks under control of relays 63I and 650 but independent of relay 632; and closes a circuit for operatively energizing the upper winding of relay l for an interval of .550 second while condenser 262 is charging. The operation of relay 634 opens the circuit for operating relay 625 and so also does the operation of relay I05. The release of relay 625 opens the talking connection between toll line TI 2 and talking conductors 96] and'962. The release .of're'lay 635 opens the circuit through the lower winding-of relay 638 and closes a circuit including the left contact of relay638 for energizing the upper winding of-relay 645. Relay .638 is heldoperated for about .030; second after relay 635 releases, byvcurrent charging condenser 636. The energization of the upper winding of relay 645 'efiects the closing of its right contact; and, since relay 634 i operated and the right contact of relay 644 is closed, the left windings of coil;629 are connected totone source 68I to initiate an impulse of current of frequency F4 of relay 638, the circuit throughthe upper winding of relay 645 isopened at the left contactof relay 638 and thelower windingof relay 64! is again energized in the circuit including the right contacts of relays 638 and 635, the contacts of relay 64! being thereby restored-to normal.

After relay 638 releases and until relay 64! re leases, the energization of the upper winding'of relay 645 is maintained by currentcharging condenser 642. Whenrelay 64! releases, the opening of the left contact of this relay deenergiz'es the upper Winding of relay 644*and' the-closing of the right contact maintains theenergization of the upper winding of relay 645. Relay 644 opens its right contactand closes its left contact thereby disconnecting the left windings "of coil 629 from tone source 68! to end theimpulse of current of frequency F4 and connecting these windings to tone source 61I'to'start an impulse of current of frequency'F3 through'coils 629 and 600 and toll line TI2. When relay 632 releases, itcauses the release off-relay 634, the current of frequency F3 being maintained for about .750 second before the release of relay 634 disconnects coil 629 from tone source 61!. Relay 180 remains operated, since it is locked under control of relays 63! and 650. When relay 634 releases, it closes the circuit for operatingrelay 625, relay 185 having released at the endof an interval of .550 second after relay 180 was operated; and the reoperation of relay 625 closes the connection between line TI2 and talking conductors96! and In response to the on-hook signal transmitted over line T!2 to the signal receiver SRI, relay 544 is operated by the current of frequency F4 and relay 534 is operated by the current of frequency F3. Relay 544 closes acircuit for opcrating relay- 545 which remains operated until relay 534 closes circuits for operating relays 535, 531 and 550. The operation. of relay 550 causes the release of cut-off relay 425; and the operation of relay 531 connects ground to conductor 53!! to energize the upper Winding of relay 510. Relay 510 opens the normally closed circuit through the upper winding of relay 515 and closes a circuit for operating relay 580. Relay 580 closes circuit for operatively energizing the upper winding of relay 585 for about .550 second, while condenser 582 is discharging. About .030 second after relay 510 operates, the current charging condenser 51! is no longer effective to maintain the operative energization of the upper winding of relay 515, the opening of the right contact of relay 515 causing an increase in the current through the upper winding of relay 560'to cause the release of relay 560; and the closingof the left contact of relay 515 being effective to cause the operation of relay 59!. Relay 592 does not operate at this time because its winding isshortcircuited by its own inner lower back contact. Relay 59! locks under control of relay 562, The opening of the right contact of relay 560causes the release of relays 56! and I05. Since relay 534 is'held operated for about .750 second, relay 510' is operated and'relay-515'released long enough to cause the release of relay 562. The release of relay 562 causesthe release of relay 59!. The release of relay I connects ground to the lower winding of relay I04, to decrease the resistance in the circuit through conductor I03 to cause the with plug P2 of cord CI)! in the manner described in'the aforementioned King et al. patent. The calling operator is thus-given an on-hook signal. When relay 534 releases at the endof the current of frequency F3, relays 535, 531 and 550 release. The release of relay 531 releases relay 516; and the release of relay 510 causes the release of relay 580 and reoperation of relay 515. With both of relays 56! and 562 released, and relay 541 being normal, relay 56!! cannot reoperate when relay 515 releases; and, therefore,

relays 5I'5I and I05 are not reoperated at the end of the signal. The release of relay 580 connects condenser 58! in series with the winding of relay 585 to reoperate relay 585 for about .550 second; and the operation of relay 585 maintains the release of cut-oil relay 425 for about .550 second after the on-hook signal ends.

When the calling operator'disconnects plug P2 from jack J2 to release the connection, relay I04 releases causing the successive release of relays I33 and H0. Relay IIO releases relays I40, I24 and I36and also releases relays I05 and I51, if operated. Relay I24 releases relays H2 and I20. Relay I36 releases relay I5! and also relay I52, if operated. The release of relay I I2 extinguishes the busy lamp L2; and the release of relay I20 reconnects the signaling conductor I61 through lighting ofrthe supervisory lamp SL2 associated resistors I2! and I22 to the windings of retard coil I50. The release of relay I40 causes the release of relays 43 I, 433 and 435 of signal transmitter STI. The release of relay 43! opens the operating circuit of slow-to-release relay 432 and closes a circuit for operating relay 434, this circuit being opened when relay 432 releases. The operation of relay 434 causes the release of cut-off relay. 425 to open the connection between line TI2 and talking conductors I6! and I62. The release of relay 435 closes a circuit including the right contact of relay 438 for energizing the upper winding of relay 445. Relay 445 thereupon reoperates, restoring its contacts to normal position to initiate an impulse of current of frequency F2 through the coils 429, 400 and toll line TI2.

' The release of relay 435 opens the circuitthrough the lower winding of relay 438, the release of relay 438' being delayed for about .030 second while condenser 436 is charging. When relay 438 releases, it opens the circuit through-the upper winding of relay 445 and closes the circuit for energizing the lower winding of relay 44! to restore the contacts of relay 44! to normal position. After relay 433 releases and until relay 44! closes its left contact, the energization of the upper winding of relay 445 is maintained b current charging condenser .442. Whenrelay 44! operates, the opening of the right contact of relay 44! deenergizes the upper winding of relay 444; and the closing of the left contact of relay 44! maintains the operative energization of the upper winding of relay 445. The deenergization of the upper winding of relay 444 disconnects tone source 48! from the right windingsof coil 429, to vend the current of frequency F2; and connects tone source 41! to those windings, to initiate an impulse of current of frequency Fl. When the slow-to-release relay 432 releases, it opens the operating'circuit of slow-to-release relay434' and closes a circuit for operating relay 450,, this circuit being traced from the winding of relay 450 through a back contact of relay 590, conductor 454,; a from contact of relay 434, a back contact of relay 432, ba'ck contact .ofrelay 43!, to ground at'a back contact of relay-450; Relay 450 closes 

